feat: emit sigil.gen_ai.tool_call_count span attribute

[alexander-akhmetov]

Set tool_call_count as an int64 span attribute on
generation spans, letting users filter by tool call count.

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Note

Low Risk
Low risk: adds a new optional span attribute derived from existing generation output without changing export formats or control flow beyond extra tag setting and test assertions.

Overview
Adds a new generation-span attribute, sigil.gen_ai.tool_call_count, emitted when tool-call parts are present in the model output (Dotnet/Go/Java/JS/Python).

Updates conformance tests across SDKs to assert the new attribute is set (and makes JS generation.output optional to support counting when present).

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[alexander-akhmetov]

bugbot review

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